Protective covers for gas sensor, gas sensor and gas sensor manufacturing method

ABSTRACT

A gas sensor comprises a cylindrical metallic housing, a sensor element having at a front end thereof a sensing portion and disposed axially in the housing with the sensing portion protruded from a front end of the housing, a sensor terminal fitted to a rear end of the sensor element to produce a signal output from the sensing portion, a first protective cover attached to a front end portion of the housing to cover the sensing portion and a second protective cover attached to a rear end portion of the housing to cover the sensor terminal. The first and second protective covers includes attachment portions attached around the front and rear end portions of the housing, respectively. At least one of the attachment portions of the first and second protective covers has a lower hardness than a remaining portion of the protective cover.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to protective covers for a gassensor, a gas sensor and a method of manufacturing a gas sensor.Hereinafter, the term “front” refers to a sensing end side with respectto the axial direction of a gas sensor, and the term “rear” refers to aside opposite to the front side.

[0002] Japanese Laid-Open Patent Publication No. 11-352095 discloses aconventional type of gas sensor as shown in FIG. 8, which comprises acylindrical metallic housing 81, a sensor element 82, a heater 83,metallic protective covers 84 and 85, a ground terminal 91, an outputterminal 92, a ground lead 93, an output lead 94, a pair of heaterterminals 95 (only one shown) and heater leads (not shown). The housing81 has a metallic shell 81 a and an inner tube 81 b fixed to each other,although it can alternatively be formed into one piece. The sensorelement 82 is disposed axially in the housing 81 with a sensing portionthereof protruded from a front end of the housing 81, and the heater 83is inserted in the sensor element 82. The ground and output terminals 91and 92 are fitted to a rear end of the sensor element 82 to makeelectrical connection between the sensor element 82 and the ground andoutput leads 93 and 94, whereas the heater terminals 95 are fitted to arear end of the heater 95 to make electrical connection between theheater 83 and the heater leads. The protective cover 84 is attached to afront end portion of the housing 81 to cover the sensing portion of thesensor element 82. The protective cover 85 is attached to a rear endportion of the housing 81 to cover the ground and output terminals 91and 92 and the heater terminals 95 with the ground and output leads 93and 94 and the heater leads projected rearwardly from the protectivecover 85.

SUMMARY OF THE INVENTION

[0003] In the above conventional type gas sensor, there is a difficultyin attaching the protective covers 84 and 85 to the housing 81.

[0004] It has been common practice to attach a protective cover to asensor housing by forming an engaging part on the protective cover,engaging the housing with the engaging part of the protective cover, andthen, welding the engaging part of the cover protective to the housing.However, stringent dimensional control is required for proper engagementbetween the housing and the engaging part of the protective cover. Theprocessing and inspection of the housing and the protective cover aretime- and labor-consuming, thereby resulting in a high manufacturingcost of the gas sensor. In the case of the protective cover being formedby drawing austenitic stainless steel, the protective cover is easilyexpansible due to elasticity. It is specifically burdensome and thuscostly to perform stringent dimensional control in the processing andinspection of such an easily expansible protective cover.

[0005] Against this backdrop, another cover attachment method isproposed in Japanese Laid-Open Patent Publication No. 11-352095, inwhich portions 84 a and 85 a of the protective covers 84 and 85 arecaulked to front and rear end portions of the housing 81 and then weldedat weld sections W1 and W2 to the housing 81. This makes it possible tosimplify the processing and inspection of the housing 81 and theprotective covers 84 and 85 without the need for stringent dimensionalcontrol and thereby possible to reduce the manufacturing cost of the gassensor.

[0006] In the above proposed method, however, the portions 84 a and 85 aof the protective covers 84 and 85 have inside diameters considerablylarger than the outside diameters of the front and rear end portions ofthe housing 81 before being caulked so that the portions 84 a and 85 aof the protective covers 84 and 85 can be easily disposed around thehousing 81. It is not easy to caulk such large-diameter portions 84 aand 85 a of the protective covers 84 and 85 to the housing 81 assuredly,as the protective covers 84 and 85 have relatively large thicknesses soas to protect the sensing portion and the terminals 91, 92 and 95 fromexternal shocks. Nevertheless, there cannot be clearance left betweenthe housing 81 and the portions 84 a and 85 a of the protective covers84 and 85 in order to weld the portions 84 a and 85 a of the protectivecovers 84 and 85 to the housing 81 securely. Accordingly, the portions84 a and 85 a of the protective covers 84 and 85 have to be caulkedthrough the application of high pressure, which causes deteriorations inthe durability of a caulking tool etc. The manufacturing cost of the gassensor cannot be then reduced sufficiently.

[0007] It is conceivable to form the protective covers 84 and 85 from arelatively soft material so that the portions 84 a and 85 a of theprotective covers 84 and 85 can be caulked without the application ofhigh pressure. This makes it possible to reduce the manufacturing costof the gas sensor to a sufficient degree. However, the protective covers84 and 85 become too weak to act as protections against the externalshocks. The gas sensor cannot attain sufficient durability for use ine.g. a vehicle.

[0008] If the protective cover 84 has a double-wall structure, therearises an additional problem that the gas sensor delays in responding toa sudden change in measurement gas.

[0009] It is therefore an object of the present invention to provideprotective covers for a gas sensor, which can be attached to a sensorhousing easily and assuredly to cover a sensor element and sensorterminals without causing the above-mentioned cost and durabilityproblems, as well as a gas sensor having such a protective cover(s) anda method of manufacturing the gas sensor.

[0010] It is also an object of the present invention to provide a gassensor with a double-wall protective cover, which shows a higherresponsivity.

[0011] According to a first aspect of the invention, there is provided aprotective cover for a gas sensor having a cylindrical metallic housingand a sensor element axially disposed in the housing with a sensingportion thereof protruded from a front end of the housing, theprotective cover including an attachment portion to be attached around afront end portion of the housing in such a manner as to cover thesensing portion with the protective cover, the attachment portion havinga lower hardness than a remaining portion of the protective cover.

[0012] According to a second aspect of the invention, there is provideda protective cover for a gas sensor having a cylindrical metallichousing, a sensor element axially disposed in the housing and having ata front end thereof a sensing portion and a sensor terminal fitted to arear end of the sensor element to produce a signal output from thesensing portion, the protective cover including an attachment portion tobe attached around a rear end portion of the housing in such a manner asto cover the sensor terminal with the protective cover, the attachmentportion having a lower hardness than a remaining portion of theprotective cover.

[0013] According to a third aspect of the invention, there is provided agas sensor, comprising: a cylindrical metallic housing; a sensor elementhaving at a front end thereof a sensing portion and disposed axially inthe housing with the sensing portion protruded from a front end of thehousing; a sensor terminal fitted to a rear end of the sensor element toproduce a signal output from the sensing portion; a first protectivecover attached to a front end portion of the housing to cover thesensing portion, the first protective cover including an attachmentportion attached around the front end portion of the housing; and asecond protective cover attached to a rear end portion of the housing tocover the sensor terminal, wherein the attachment portion has a lowerhardness than a remaining portion of the first protective cover.

[0014] According to a fourth aspect of the invention, there is provideda gas sensor, comprising: a cylindrical metallic housing; a sensorelement having at a front end thereof a sensing portion and disposedaxially in the housing with the sensing portion protruded from a frontend of the housing; a sensor terminal fitted to a rear end of the sensorelement to produce a signal output from the sensing portion; a firstprotective cover attached to a front end portion of the housing to coverthe sensing portion; and a second protective cover attached to a rearend portion of the housing to cover the sensor terminal, the secondprotective cover including an attachment portion attached around therear end portion of the housing, wherein the attachment portion has alower hardness than a remaining portion of the second protective cover.

[0015] According to a fifth aspect of the invention, there is provided agas sensor, comprising: a cylindrical metallic housing; a sensor elementhaving at a front end thereof a sensing portion and disposed axially inthe housing with the sensing portion protruded from a front end of thehousing; a sensor terminal fitted to a rear end of the sensor element toproduce a signal output from the sensing portion; a first protectivecover attached to a front end portion of the housing to cover thesensing portion; and a second protective cover attached to a rear endportion of the housing to cover the sensor terminal, wherein the firstprotective cover has: an outer cover member including an attachmentportion attached around the rear end portion of the housing and an outercircumferential wall, the outer circumferential wall having outer gasinlet holes formed therein to feed a measurement gas into the outercover member; and an inner cover member fixed in the outer cover memberand including an inner circumferential wall held in parallel with theouter circumferential wall and at a rear end thereof a wall edge bent tothe outer circumferential wall, the wall edge having inner gas inletholes formed therein to feed the measurement gas from the outer covermember into the inner cover member.

[0016] According to a sixth aspect of the invention, there is provided amethod of manufacturing a gas sensor, comprising: disposing a sensorelement in a cylindrical metallic housing, the sensor element having ata front end thereof a sensing portion protruded from a front end of thehousing; forming a protective cover with an attachment portion; making ahardness of the attachment portion lower than that of a remainingportion of the protective cover; and attaching the attachment portion ofthe protective cover around a front end portion of the housing in such amanner as to cover the sensing portion with the protective cover.

[0017] According to a seventh aspect of the invention, there is provideda method of manufacturing a gas sensor, comprising: disposing a sensorelement in a cylindrical metallic shell, the sensor element having at afront end thereof a sensing portion; fitting a sensor terminal to a rearend of the sensor element for signal output from the sensing portion;forming a protective cover with an attachment portion; making a hardnessof the attachment portion lower in hardness than that of a remainingportion of the protective cover; and attaching the attachment portion ofthe protective cover around a rear end portion of the housing in such amanner as to cover the sensor terminal with the protective cover.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a sectional view of an oxygen sensor according to oneembodiment of the present invention.

[0019]FIG. 2 is a process chart for manufacturing the oxygen sensor.

[0020]FIG. 3A is a sectional view of a terminal cover of the oxygensensor, after the process of forming.

[0021]FIG. 3B is a sectional view of the terminal cover of FIG. 3A,during the process of hardness control.

[0022]FIG. 4 is a partially cutaway view of a sensor-element cover ofthe oxygen sensor.

[0023]FIG. 5 is a sectional view showing how the oxygen sensor comesassembled.

[0024]FIG. 6 is a sectional view of the oxygen sensor in a conditionwhere the terminal cover is not yet caulked.

[0025]FIG. 7 is a graph showing the relationship between a hardness anda location on the terminal cover.

[0026]FIG. 8 is a sectional view of a conventional type gas sensor.

DESCRIPTION OF THE EMBODIMENTS

[0027] The present invention will be described below with reference tothe drawings. Although the present invention can be applied to variouskinds of sensors including a gas concentration sensor that detects theconcentration of a specific component of a measurement gas (such as anoxygen sensor, a HC sensor or a NOx sensor) and a gas temperature sensorthat detects the temperature of a measurement gas, the followingembodiment specifically refers to an oxygen sensor.

[0028] As shown in FIG. 1, an oxygen sensor according to one embodimentof the present invention comprises a sensor element 1, a ground terminal2, an output terminal 3, a ground lead 4, an output lead 5, a heater 6,a pair of heater terminals 7 (only one shown), heater leads 8 (only oneshown), a cylindrical metallic shell 14 (as a sensor housing) andmetallic protective covers 15 and 16.

[0029] The sensor element 1 has at a front end thereof a sensing portionto detect the concentration of oxygen in a measurement gas. The sensorelement 1 includes a cylindrical element body having its front endclosed, an inner electrode layer formed on an inner surface of the body,and an outer electrode layer formed on an outer surface of the body. Theelement body is made of a solid electrolyte e.g. zirconium oxide (ZrO₂)stabilized with yttrium oxide (Y₂O₃) or calcium oxide (CaO), whereas theinner and outer electrode layers are made of e.g. platinum (Pt).

[0030] The ground and output terminals 2 and 3 are fitted to a rear endof the sensor element 1. The ground terminal 2 makes electricalconnection between the outer electrode layer of the sensor element 1 andthe ground lead 4 to establish a ground for the sensor element 1, andthe output terminal 3 makes electrical connection between the innerelectrode layer of the sensor element 1 and the output lead to retrievean output signal from the sensing portion of the sensor element 1.

[0031] The heater 6 is inserted in the body of the sensor element 1, andthe heater terminals 7 are fitted to a rear end of the heater 6. Theheater leads 8 are connected to the respective heater terminals 7 forenergization of the heater 6.

[0032] The sensor element 1 is axially disposed in the metallic shell 14with the sensing portion protruded from a front end of the metallicshell 14. Metallic packings 9 a and 9 b, a ceramic holder 10, a sealant11 made of e.g. talc, a ceramic sleeve 12 and a metallic ring 13 areinterposed between the sensor element 1 and the metallic shell 14. Arear end portion of the metallic shell 14 is caulked radially inwardlyto the sensor element 1 via the ceramic sleeve 12, so that the sensorelement 1 is held in the metallic shell 14 while being kept insulatedfrom the metallic shell 14.

[0033] The metallic shell 14 includes front and rear cylindrical bossend portions 14 a and 14 b, a flange portion 14 c, and a male-threadportion 14 d formed between the boss end portion 14 a and the flangeportion 14 c. A gasket G is fitted around the metallic shell 14 at aposition between the flange portion 14 c and the male-thread portion 14d.

[0034] The protective cover 15 is attached to the boss end portion 14 aof the metallic shell 14 to cover the sensing portion of the sensorelement 1. Preferably, the protective cover 15 has a double-wallstructure with an outer cover member 15 a and an inner cover member 15 bfixed in the outer cover member 15 a as shown in FIG. 4.

[0035] The outer cover member 15 a has a circumferential wall 15 w and acylindrical attachment portion 15 c formed integral with thecircumferential wall 15 w at a rear end of the outer cover member 15 a.The attachment portion 15 c is disposed around the boss end portion 14 aof the metallic shell 14 to form an overlap O1 between the attachmentportion 15 c and the boss end portion 14 a, and is attached to the bossend portion 14 a by caulking and welding at the overlap O1.

[0036] The inner cover member 15 b has a circumferential wall 15 x heldin parallel to the circumferential wall 15 w with a predetermined spaceleft therebetween, and at a rear end thereof a wall edge 15 y broughtinto contact with an inner surface of the circumferential wall 15 w.

[0037] In the present embodiment, the attachment portion 15 c has alower hardness than that of a remaining portion of the cover 15(including the circumferential walls 15 x and 15 y and the wall edge 15y), so that the attachment portion 15 c can be easily and assuredlycaulked to the boss end portion 14 a of the metallic shell 14 withoutclearance left between the attachment portion 15 c and the boss endportion 14 a. It is preferable that the attachment portion 15 c has aVickers hardness of 100 to 200 and the remaining portion of the cover 15has a Vickers hardness of 300 to 400. If the Vickers hardness of theattachment portion 15 c is lower than 100, the attachment portion 15 ccannot secure sufficient strength after the caulking and welding. If theVickers hardness of the attachment portion 15 c is higher than 200, itis difficult to caulk the attachment portion 15 c to the boss endportion 14 a assuredly while making the protective cover 15 with arelatively large thickness. The remaining portion of the cover 15becomes too weak to act as a protection against external shocks and toattain sufficient durability if the Vickers hardness of the remainingportion of the cover 15 is lower than 300. If the Vickers hardness ofthe remaining portion of the cover 15 is higher than 400, the remainingportion becomes susceptible to cracks due to its brittleness anddifficult to form. Herein, the Vickers hardness is defined as an averageof measurement values obtained in Vickers hardness test with a load of300 g applied at established axial intervals (e.g. every 0.5 mm in thepresent embodiment) to an axial section through the protective cover 15at half of its radial thickness.

[0038] Each of the attachment portion 15 c and the remaining portion ofthe cover 15 preferably has a thickness of 0.4 mm or larger so as toattain sufficient shock resistance. The attachment portion 15 c and theremaining portion of the cover 15 are desirably 1.0 mm or smaller inthickness in consideration of formability. In a case where theprotective cover 15 has the above double-wall structure, it ispreferable that the outer cover member 15 a has a thickness of 0.4 mm to1.0 mm and the inner cover member 15 b has a thickness of less than 0.4mm. The inner cover member 15 b does not necessarily require so highdurability as the outer cover member 15 a. Accordingly, the inner covermember 15 b can be made thinner than the outer cover member 15 a fordownsizing of the oxygen sensor. In the present embodiment, the outerand inner cover members 15 a and 15 b are made 0.5 mm and 0.3 mm inthickness, respectively.

[0039] The attachment portion 15 c and the remaining portion of thecover 15 are preferably made of austenitic stainless steel such asSUS304L or SUS310.

[0040] In a case where the protective cover 15 has a total length of 20to 50 mm, the attachment portion 15 c preferably has a length of 5 to 7mm. If the length of the attachment portion 15 c is less than 5 mm, itis difficult to caulk and weld the attachment portion 15 c to the bossend portion 14 a. If the length of the attachment portion 15 c is morethan 7 mm, the remaining portion of the cover 15 becomes too short toact as a protection against external shocks and to attain sufficientdurability. In the present embodiment, the protective cover 15 and itsattachment portion 15 a are made 20 mm and 5 mm in length, respectively.Further, the inner cover member 15 b has a length shorter than that ofthe outer cover member 15 a so that the protective cover 15 is attachedto the metallic shell 14 only at the attachment portion 15 c of theouter cover member 15 b. It is easier to caulk and weld the attachmentportion 15 c directly to the metallic shell 14 than to caulk and weldthe attachment portion 15 c via the inner cover member 15 b to themetallic housing 14.

[0041] Gas inlet holes 15 d and 15 e are formed in the circumferentialwall 15 w of the outer cover member 15 a and the wall edge 15 y of theinner cover member 15 b, respectively, so as to feed the measurement gasto the sensing portion of the sensor element 1. In order for the walledge 15 y to be easily formed with the gas inlet holes 15 e, cuts 15 zare made in the wall edge 15 y, and then, the wall edge 15 y is bent tothe circumferential wall 15 w in such a manner as to bring the wall edge15 y into contact with the inner surface of the circumferential wall 15w. With this, the gas inlet holes 15 e are easily defined by the cuts 15z of the wall edge 15 y and the circumferential wall 15 w so as to facetoward a gas passage between the circumferential walls 15 x and 15 y.

[0042] Gas outlet holes 15 h and 15 k are formed in bottoms of the innerand outer cover members 15 b and 15 a, respectively, to discharge themeasurement gas out of the protective cover 15. In the presentembodiment, the outer and inner cover members 15 a and 15 b are joinedtogether by e.g. spot welding at positions around the gas outlet holes15 k and 15 h in order to prevent the gas from flowing directly from thegas inlet hole 15 d to the gas outlet hole 15 k.

[0043] In the above double-wall structure of the protective cover 15,the measurement gas flows into the outer cover member 15 a via the gasinlet holes 15 d, flows rearwardly through the gas passage between thecircumferential walls 15 x and 15 w, flows into the inner cover member15 b via the gas inlet holes 15 e, flows through a gas passage betweenthe wall edge 15 y and the boss end portion 14 a, flows frontwardlywithin the inner cover member 15 b to be supplied to the sensingportion, and then, flows out of the protective cover 15 through the gasoutlet holes 15 h and 15 k. As the gas inlet holes 15 e face toward thegas passage between the circumferential walls 15 x and 15 y, themeasurement gas flows into the inner cover member 15 b without a largeflow resistance. Further, the measurement gas flows within the innercover member 15 b in such a manner as to force a gas holdup in the covermember 15 b toward the gas outlet holes 15 h and 15 k. Gas replacementcan be therefore performed smoothly within the protective cover 15 bysuch a zigzag flow. Even if there is a sudden change in oxygenconcentration, the oxygen sensor becomes able to provide a quickresponse to such a sudden change. Namely, the oxygen sensor of thepresent embodiment can attain higher responsivity than a conventionaloxygen sensor with a double-wall sensor cover.

[0044] Herein, there is a possibility that the flow of gas to thesensing portion of the sensor element 1 may be prevented due to a waterdrop accumulated in the gas passage between the wall edge 15 y and theboss end portion 14 a. In order to avoid such a possibility, it ispreferable to make the thickness of a front edge of the boss end portion14 a smaller than a length of the wall edge 15 y (i.e. a distancebetween the circumferential walls 15 x and 15 w of the cover members 15b and 15 a), or to form the cuts 15 z over the wall edge 15 y and thecircumferential wall 15 x.

[0045] The protective cover 16 is attached to the boss end portion 14 bof the metallic shell 14 to cover the ground and output terminals 2 and3 and the heater terminals 7 with the ground and output leads 4 and 5and the heater leads 8 projected rearwardly from the protective cover16.

[0046] The protective cover 16 includes at a front end thereof acylindrical attachment portion 16 a. The attachment portion 16 a isdisposed around the boss end portion 14 b to form an overlap O2 betweenthe attachment portion 16 a and the boss end portion 14 b, and isattached to the boss end portion 14 b by caulking and welding at theoverlap O2.

[0047] In the present embodiment, the attachment portion 16 a has alower hardness than that of a remaining portion 16 b of the cover 16, sothat the attachment portion 16 a can be easily and assuredly caulked tothe boss end portion 14 b of the metallic shell 14 without clearanceleft between the attachment portion 16 a and the boss end portion 14 b.It is preferable that the attachment portion 16 a has a Vickers hardnessof 100 to 200 and the remaining portion 16 b has a Vickers hardness of300 to 400. If the Vickers hardness of the attachment portion 16 a islower than 100, the attachment portion 16 a cannot attain sufficientstrength after the caulking and welding. If the Vickers hardness of theattachment portion 16 a is higher than 200, it is difficult to caulk theattachment portion 16 a to the boss end portion 14 b assuredly whilemaking the protective cover 16 with a relatively large thickness. Theremaining portion 16 b becomes too weak to act as a protection againstexternal shocks and to attain sufficient durability if the Vickershardness of the remaining portion 16 b is lower than 300. If the Vickershardness of the remaining portion 16 b is higher than 400, the remainingportion 16 b becomes susceptible to cracks due to its brittleness anddifficult to form. Herein, the Vickers hardness is defined in the sameway as above i.e. as an average of measurement values obtained inVickers hardness test with a load of 300 g applied at established axialintervals (e.g. every 0.5 mm) to an axial section through the protectivecover 16 at half of its radial thickness.

[0048] Each of the attachment portion 16 a and the remaining portion 16b of the cover 16 preferably has a thickness of 0.4 mm or larger so asto attain sufficient shock resistance. The thickness of each of theattachment portion 16 a and the remaining portion 16 b is desirably 1.0mm or smaller in consideration of formability. In the presentembodiment, both of the attachment portion 16 a and the remainingportion 16 b are made 0.5 mm in thickness.

[0049] The attachment portion 16 a and the remaining portion 16 b of thecover 16 are preferably made of austenitic stainless steel such asSUS304L or SUS310.

[0050] In a case where the protective cover 16 has a total length of 20to 50 mm, the attachment portion 16 a is preferably made 5 to 7 mm inlength. If the length of the attachment portion 16 a is less than 5 mm,it is difficult to caulk and weld the attachment portion 16 a to theboss end portion 14 b. If the length of the attachment portion 16 a ismore than 7 mm, the remaining portion 16 b of the cover 16 becomes tooshort to act as a protection against external shocks and to attainsufficient durability. In the present invention, the protective cover 16and its attachment portion 16 a are made 47 mm and 6 mm in length,respectively.

[0051] A ceramic separator 18 with a flange portion 18 a is fixed in theprotective cover 16 a via a leaf spring 17 so that the separator 18surrounds the connection between the ground terminal 2 and the lead 4,the connection between the output terminal 3 and the lead 5 as well asthe respective connections between the heater terminals 7 and the leads8.

[0052] A rubber grommet 19 is fix in a rear end of the protective cover16 with the sensor leads 4 and 5 and the heater leads 8 passingtherethrough. The grommet 19 has a through hole 19 a at a centerthereof, and a filter unit 20 is disposed in the hole 19 a. The filterunit 20 has a cylindrical metallic support 20 a and a filter 20 battached to cover a rear end face and a circumferential portion of thesupport 20 a. The filter 20 b is made of e.g. polytetrafluoroethylene(PTFE), and removes moisture from the air so that dehydrated air issupplied to the inside of the protective cover 16.

[0053] The above-structured oxygen sensor can be manufactured by thefollowing procedure according to one embodiment of the presentinvention.

[0054] Firstly, all the components of the oxygen sensor (including thesensor element 1, the ground terminal 2, the output terminal 3, theground lead 4, the output lead 5, the heater 6, the heater terminals 7,the heater leads 8, the packings 9 a and 9 b, the holder 10, the sealant11, the sleeve 12, the ring 13, the metallic shell 4, the protectivecovers 15 and 16, the spring 17, the separator 18, the grommet 19 andthe filter unit 20) are prepared. In particular, the protective covers15 and 16 can be prepared in the following forming and hardness controlprocess steps (as in steps S1 and S2 of FIG. 2).

[0055] To prepare the protective cover 16, a plate or pipe of austeniticstainless steel is subjected to drawing to form a tubular article 31having a small-diameter part 31 a and a large-diameter part 31 b formedintegral with the small-diameter part 31 a as shown in FIG. 3A. Fourradially-inward protrusions 31 c are formed by drawing on thesmall-diameter part 31 a so as to be engageable with the flange portion18 a of the separator 18. Then, only an open-end side of thelarge-diameter part 31 b (which corresponds to the attachment portion 16a of the protective cover 16) is annealed by placing a high-frequencycoil 40 around the open-end side of the large-diameter part 31 b asshown in FIG. 3B, supplying the coil 40 with a high-frequency current tosubject the open-end side of the part 31 b to high-frequency inductionheating at about 700° C., and cooling the annealed open-end side of thepart 31 b in the air. Alternatively, the annealing can be done by usingan electron beam. With the annealing, the attachment portion 16 a ismade lower in hardness than the remaining portion 16 b. It is preferableto control the Vickers hardness of the attachment portion 16 a to withinthe range of 100 to 200 and to control the Vickers hardness of theremaining portion 16 b to within the range of 100 to 200 as describedabove. Further, the attachment portion 16 a has an inside diameterconsiderably larger than an outside diameter of the boss end portion 14b of the metallic shell 14 so that the attachment portion 16 a can beeasily arranged around the boss end portion 14 b.

[0056] The protective cover 15 can be prepared in the same manner. Withthe annealing, the attachment portion 15 c is also made lower inhardness than the remaining portion of the cover 15 at relatively lowcost. As described above, it is preferable to control the Vickershardness of the attachment portion 15 c to within the range of 100 to200 and to control the Vickers hardness of the remaining portion of thecover 15 to within the range of 300 to 400. Further, the attachmentportion 15 c has an inside diameter larger than an outside diameter ofthe boss end portion 14 a of the metallic shell 14 so that theattachment portion 15 c can be easily arranged around the boss endportion 14 a.

[0057] Next, the prepared components are combined into first and secondsubassemblies.

[0058] The first subassembly is provided by: passing the leads 4, 5 and8 through the grommet 19, and then, through the separator 18; connectingthe ground and output terminals 2 and 3 to the leads 4 and 5,respectively; and connecting the heater 6 to the leads 8 via therespective heater terminals 7; putting the leaf spring 17 on a front endportion of the separator 18; inserting the separator 18 in theprotective cover 16 until the flange portion 18 a of the separator 18abuts on the protrusions 31 c; and inserting the grommet 19 in the rearend of the cover 16.

[0059] The second subassembly is provided by: inserting the packing 9 a,the holder 19 and the packing 9 b in the metallic shell 14 in order ofmention; placing the sensor element 1 in the metallic shell 14 so as tobe surrounded by the packing 9 a, the holder 10 and the packing 9 b;arranging the sealant 11, the sleeve 12 and the ring 13 between thesensor element 1 and the metallic shell 14; caulking the rear endportion of the metallic shell 14 to the sensor element 1 via the sleeve12; putting the protective cover 15 on the metallic shell 14 so that theattachment portion 15 c circumferentially surrounds the boss end portion14 a to form the overlap O1; caulking the attachment portion 15 c to theboss end portion 14 a (as in step S3 of FIG. 2); and welding theattachment portion 15 c and the boss end portion 14 a together (as instep S4 of FIG. 2). In the present embodiment, the attachment portion 15c is circumferentially caulked to the boss end portion 14 a (i.e. theoverlap O1 between the attachment portion 15 c is caulked radiallyinwardly) in order to perform the subsequent welding step withoutdifficulty and thereby fix the protective cover 15 to the metallic shell14 securely. The caulking can be done by using a caulking tool equippedwith a roller or six or more punches. Preferably, a caulking tool witheight or more punches is used so that the attachment portion 15 c isassuredly caulked to the boss end portions 14 throughout itscircumference by surrounding the attachment portion 15 c with thepunches. The attachment portion 15 c is also circumferentially welded tothe boss end portion 14 a (i.e. a circumferential weld section W1 isformed in the overlap O1 between the attachment portion 15 c and theboss end portion 14 a) in order to fix the protective cover 15 to themetallic housing 14 securely, although the attachment portion 15 c canalternatively be joined to the boss portion 14 a by spot welding. Forease of welding, it is preferable to weld the attachment portion 15 ccircumferentially by using a laser beam.

[0060] While the first subassembly is supported with chucks CH, theheater 6 of the second subassembly is inserted into the body of thesensor element 1 of the first subassembly until the protective cover 16of the first subassembly abuts on the flange portion 14 c of themetallic shell 14 of the second subassembly. While inserting the heater6 into the body of the sensor element 1, the electrical connection ismade between the ground terminal 2 and the outer electrode layer of thesensor element 1 and between the output terminal 3 and the innerelectrode layer of the sensor element 1. At this time, the oxygen sensorcomes assembled as shown in FIG. 6, with a radial space left between theattachment portion 16 a of the protective cover 16 and the boss endportion 14 b of the metallic shell 14.

[0061] Then, the attachment portion 16 a is caulked and welded to theboss end portion 14 b (as in steps S3 and S4 of FIG. 2). In order toperform the subsequent welding step without difficulty and thereby fixthe protective cover 15 to the metallic shell 14 securely, theattachment portion 16 a is circumferentially caulked to the boss endportion 14 b (i.e. the overlap O2 between the attachment portion 16 aand the boss end portion 14 b is caulked radially inwardly). Thecaulking can be done by using a caulking tool equipped with a roller orsix or more punches. Preferably, a caulking tool with eight or morepunches is used so that the attachment portion 16 a is assuredly caulkedto the boss end portion 14 b throughout its circumference by surroundingthe attachment portion 16 a with the punches. The attachment portion 16a is also circumferentially welded to the boss end portion 14 b (i.e. acircumferential weld section W2 is formed in the overlap O2 between theattachment portion 16 a and the boss end portion 14 b) in order to fixthe protective cover 16 to the metallic housing 14 securely, althoughthe attachment portion 16 a can alternatively be joined to the boss endportion 14 b by spot welding. For ease of welding, it is preferable toweld the attachment portion 16 a circumferentially by using a laserbeam.

[0062] Finally, the protective cover 16 is caulked radially inwardly ata position axially corresponding to the leaf spring 17 so as to hold theseparator 18 with the flange portion 18 a sandwiched between theprotrusions 31 c of the cover 16 and the leaf spring 17, and is caulkedat a position axially corresponding to the grommet 18. The oxygen sensoris then completed as shown in FIG. 1.

[0063] For use of the oxygen sensor, the oxygen sensor is mounted ine.g. an exhaust pipe of a vehicle. When the measurement gas is suppliedto the sensing portion of the sensor element 1 via the gas inlet holes15 d and 15 e, there arises an electromotive force between the inner andouter electrode layers of the sensor element 1 in accordance with theconcentration of oxygen in the measurement gas. The electromotive forceis outputted as a signal from the inner electrode layer through theoutput terminal 3 and the output lead 5, whereas the outer electrodelayer is grounded through the ground terminal 2 and the ground lead 4.During the measurement of oxygen concentration, the protective cover 15protects the sensing portion from water or oil drops, and the protectivecover 16 protects the sensing portion and the terminals 2, 3 and 6 fromflying gravel etc. Further, the filter unit 20 feeds dehydrated air intothe protective cover 16 by means of the filter 20 b so as to set astandard for the output signal. The heater 6 is energized through theheater terminals 7 and the leads 8 for quick actuation of the sensingportion.

[0064] As described above, the attachment portions 15 c and 16 a of theprotective covers 15 and 16 are made lower in hardness. The attachmentportions 15 c and 16 a can be therefore caulked to the boss end portions14 a and 14 b of the metallic housing 14 easily and assuredly withoutclearance therebetween, even when the protective covers 15 and 16 haverelatively large thicknesses so as to protect the sensor element 1 andthe terminals 2, 3 and 7 from external shocks and when the attachmentportions 15 c and 16 a have relatively large inside diameters thanoutside diameters of the boss end portions 14 a and 14 b so as to beeasily arranged around the boss end portions 14 a and 14 b. Even in thecase of the protective covers 15 and 16 being formed by drawingaustenitic stainless steel, there is no need to perform stringentdimensional control for engagement between the attachment portion 15 cand the boss end portion 14 a and between the attachment portion 16 aand the boss end portion 14 b. Also, there is no need to apply highpressure to caulk the attachment portions 15 c and 16 a, therebyavoiding deteriorations in the durability of the caulking tool etc. Theabove assuredly caulked attachment portions 15 c and 16 a can be weldedto metallic shell 4 easily and assuredly at sufficient weld strength.The protective covers 15 and 16 properly work to protect the sensorelement 1 and the terminals 2, 3 and 7 from external shocks because theprotective covers 15 and 16 have relatively large thickness and theremaining portions of the covers 15 and 16 are higher in hardness. Itbecomes possible according to the present embodiment of the invention tosimplify the manufacturing process of the oxygen sensor for reduction inmanufacturing cost while securing sufficient durability for the oxygensensor under use in the vehicle.

[0065] The present invention will be specifically illustrated in moredetail by way of the following examples. However, it should be notedthat the following example is only illustrative and not intended tolimit the present invention thereto.

EXAMPLE

[0066] A protective cover (corresponding to the cover 16 in the aboveembodiment) was formed from SUS304L with the following dimensions asshown in FIG. 3A. Only an attachment portion of the cover was annealedusing a high-frequency coil as shown in FIG. 3B. [Dimensions] Length ofthe cover:  47 mm Length of the attachment portion:   6 mm Thickness ofthe cover: 0.5 mm

[0067] The prepared protective cover showed hardness characteristics asshown in FIG. 7. Namely, the attachment portion had a Vickers hardnessof 100 to 200 mm, and the remaining portion of the cover had a Vickershardness of 300 to 400 (on average).

[0068] An oxygen sensor was assembled using the prepared protectivecover in compliance with the above-described embodiment. The caulkingwas done by using a caulking tool with eight punches, and the weldingwas done by laser welding. The attachment portion of the cover wascaulked easily and assuredly without causing deteriorations in thedurability of the punches etc. Further, the protective cover was able tosatisfactorily act as a protection. It is thus possible to reduce themanufacturing cost of the oxygen sensor while securing sufficientdurability for the sensor by the use of the protective cover of EXAMPLE.

Comparative Example 1

[0069] The same procedure as in EXAMPLE was repeated, except the entirecover was not given annealing. As shown in FIG. 7, the preparedprotective cover had a Vickers hardness of 300 to 400 throughout itslength. In the assembling of the oxygen sensor, the attachment portionof the cover was caulked by the application of increased pressure tocause deteriorations in the durability of the punches etc. It is thusconcluded that the manufacturing cost of the oxygen sensor cannot bereduced sufficiently by the use of the protective cover of COMPARATIVEEXAMPLE 1.

Comparative Example 2

[0070] The same procedure as in EXAMPLE was repeated, except that theentire cover was given annealing. The prepared protective cover had aVickers hardness of 200 or lower throughout its length, as shown in FIG.7. In the assembling of the oxygen sensor, the attachment portion of theprotective cover was caulked without causing deteriorations in thedurability of the punches etc. However, the protective cover was tooweak to act as a protection against external shocks. It is thusconcluded that the oxygen sensor does attain sufficient durability bythe use of the protective cover of COMPARATIVE EXAMPLE 2 although themanufacturing cost of the sensor can be reduced.

[0071] The entire contents of Japanese Patent Application No.2002-239354 (filed on Aug. 20, 2002) are herein incorporated byreference.

[0072] Although the present invention has been described with referenceto a specific embodiment of the invention, the invention is not limitedto the above-described embodiment. Various modification and variation ofthe embodiment described above will occur to those skilled in the art inlight of the above teaching. For example, either one of the attachmentportions 15 c and 16 a of the protective covers 15 and 16 may have alower hardness than that of the remaining portion. Further, the sensorhousing may have a cylindrical metallic shell and an inner tube fixed toeach other by e.g. caulking. The scope of the invention is defined withreference to the following claims.

What is claimed is:
 1. A protective cover for a gas sensor having acylindrical metallic housing and a sensor element axially disposed inthe housing with a sensing portion thereof protruded from a front end ofthe housing, the protective cover including an attachment portion to beattached around a front end portion of the housing in such a manner asto cover the sensing portion with the protective cover, the attachmentportion having a lower hardness than a remaining portion of theprotective cover.
 2. The protective cover according to claim 1, whereinthe attachment portion of the protective cover has a Vickers hardness of100 to
 200. 3. The protective cover according to claim 1, wherein theremaining portion of the protective cover has a Vickers hardness of 300to
 400. 4. The protective cover according to claim 1, wherein each ofthe attachment portion and the remaining portion of the protective coverhas a thickness of 0.4 mm or larger.
 5. The protective cover accordingto claim 1, wherein the attachment portion and the remaining portion ofthe protective cover are made of austenitic stainless steel.
 6. Theprotective cover according to claim 1, wherein only the attachmentportion of the protective cover has been annealed.
 7. The protectivecover according to claim 1, wherein the protective cover comprises anouter cover member and an inner cover member fixed in the outer covermember to cover the sensing portion, and the attachment portion isformed on the outer cover member.
 8. The protective cover according toclaim 7, wherein the outer cover member has a thickness of 0.4 mm orlarger, and the inner cover member has a smaller thickness than that ofthe outer cover member.
 9. A protective cover for a gas sensor having acylindrical metallic housing, a sensor element axially disposed in thehousing and having at a front end thereof a sensing portion and a sensorterminal fitted to a rear end of the sensor element to produce a signaloutput from the sensing portion, the protective cover including anattachment portion to be attached around a rear end portion of thehousing in such a manner as to cover the sensor terminal with theprotective cover, the attachment portion having a lower hardness than aremaining portion of the protective cover.
 10. The protective coveraccording to claim 9, wherein the attachment portion of the protectivecover has a Vickers hardness of 100 to
 200. 11. The protective coveraccording to claim 9, wherein the remaining portion of the protectivecover has a Vickers hardness of 300 to
 400. 12. The protective coveraccording to claim 9, wherein each of the attachment portion and theremaining portion of the protective cover has a thickness of 0.4 mm orlarger.
 13. The protective cover according to claim 9, wherein theattachment portion and the remaining portion of the protective cover aremade of austenitic stainless steel.
 14. The protective cover accordingto claim 9, wherein only the attachment portion of the protective coverhas been annealed.
 15. A gas sensor, comprising: a cylindrical metallichousing; a sensor element having at a front end thereof a sensingportion and disposed axially in the housing with the sensing portionprotruded from a front end of the housing; a sensor terminal fitted to arear end of the sensor element to produce a signal output from thesensing portion; a first protective cover attached to a front endportion of the housing to cover the sensing portion, the firstprotective cover including an attachment portion attached around thefront end portion of the housing; and a second protective cover attachedto a rear end portion of the housing to cover the sensor terminal,wherein the attachment portion has a lower hardness than a remainingportion of the first protective cover.
 16. The gas sensor according toclaim 15, wherein the attachment portion of the first protective coveris attached around the front end portion of the housing by caulking andwelding.
 17. The gas sensor according to claim 15, wherein theattachment portion of the first protective cover is circumferentiallywelded to the front end portion of the housing.
 18. The gas sensoraccording to claim 15, wherein the first protective cover comprises anouter cover member and an inner cover member fixed in the outer covermember to cover the sensing portion, and the attachment portion isformed on the outer cover member.
 19. The gas sensor according to claim18, wherein the outer cover member includes an outer circumferentialwall; the outer circumferential wall has outer gas inlet holes formedtherein to feed a measurement gas into the outer cover member; the innercover member includes an inner circumferential wall held in parallelwith the outer circumferential wall and at a rear end thereof a walledge bent to the outer circumferential wall; and the wall edge of theinner cover member has inner gas inlet holes formed therein to feed themeasurement gas from the outer cover member into the inner cover member.20. The gas sensor according to claim 19, wherein the wall edge of theinner cover member has cuts formed therein to define the inner gas inletholes.
 21. The gas sensor according to claim 18, wherein the outer covermember has a thickness of 0.4 mm or greater, and the inner cover memberhas a smaller thickness than that of the outer cover member.
 22. A gassensor, comprising: a cylindrical metallic housing; a sensor elementhaving at a front end thereof a sensing portion and disposed axially inthe housing with the sensing portion protruded from a front end of thehousing; a sensor terminal fitted to a rear end of the sensor element toproduce a signal output from the sensing portion; a first protectivecover attached to a front end portion of the housing to cover thesensing portion; and a second protective cover attached to a rear endportion of the housing to cover the sensor terminal, the secondprotective cover including an attachment portion attached around therear end portion of the housing, wherein the attachment portion has alower hardness than a remaining portion of the second protective cover.23. The gas sensor according to claim 22, wherein the attachment portionof the second protective cover is attached around the rear end portionof the housing by caulking and welding.
 24. The gas sensor according toclaim 24, wherein the attachment portion of the second protective coveris circumferentially welded to the rear end portion of the housing. 25.A gas sensor, comprising: a cylindrical metallic housing; a sensorelement having at a front end thereof a sensing portion and disposedaxially in the housing with the sensing portion protruded from a frontend of the housing; a sensor terminal fitted to a rear end of the sensorelement to produce a signal output from the sensing portion; a firstprotective cover attached to a front end portion of the housing to coverthe sensing portion; and a second protective cover attached to a rearend portion of the housing to cover the sensor terminal, wherein thefirst protective cover has: an outer cover member including anattachment portion attached around the rear end portion of the housingand an outer circumferential wall, the outer circumferential wall havingouter gas inlet holes formed therein to feed a measurement gas into theouter cover member; and an inner cover member fixed in the outer covermember and including an inner circumferential wall held in parallel withthe outer circumferential wall and at a rear end thereof a wall edgebent to the outer circumferential wall, the wall edge having inner gasinlet holes formed therein to feed the measurement gas from the outercover member into the inner cover member.
 26. The gas sensor accordingto claim 25, wherein the attachment portion has a lower hardness than aremaining portion of the first protective cover.
 27. The gas sensoraccording to claim 25, wherein the wall edge of the inner cover memberhas cuts formed therein to define the inner gas inlet holes.
 28. The gassensor according to claim 25, wherein the outer cover member has alarger thickness than that of the inner cover member.
 29. A method ofmanufacturing a gas sensor, comprising: disposing a sensor element in acylindrical metallic housing, the sensor element having at a front endthereof a sensing portion protruded from a front end of the housing;forming a protective cover with an attachment portion; making a hardnessof the attachment portion lower than that of a remaining portion of theprotective cover; and attaching the attachment portion of the protectivecover around a front end portion of the housing in such a manner as tocover the sensing portion with the protective cover.
 30. The methodaccording to claim 29, wherein the attaching comprises: arranging theattachment portion of the protective cover around the front end portionof the housing; caulking the attachment portion of the protective coverto the front end portion of the housing; and circumferentially weldingthe attachment portion of the protective cover to the front end portionof the housing.
 31. The method according to claim 29, wherein theprotective cover is formed by drawing austenitic stainless steel. 32.The method according to claim 29, wherein the hardness of the attachmentportion is made lower by annealing.
 33. The method according to claim32, wherein the annealing is performed by high-frequency inductionheating.
 34. The method according to claim 30, wherein the welding islaser welding.
 35. A method of manufacturing a gas sensor, comprising:disposing a sensor element in a cylindrical metallic shell, the sensorelement having at a front end thereof a sensing portion; fitting asensor terminal to a rear end of the sensor element for signal outputfrom the sensing portion; forming a protective cover with an attachmentportion; making a hardness of the attachment portion lower in hardnessthan that of a remaining portion of the protective cover; and attachingthe attachment portion of the protective cover around a rear end portionof the housing in such a manner as to cover the sensor terminal with theprotective cover.
 36. The method according to claim 36, wherein theattaching comprises: arranging the attachment portion of the protectivecover around the rear end portion of the housing; caulking theattachment portion of the protective cover to the rear end portion ofthe housing; and circumferentially welding the attachment portion of theprotective cover to the rear end portion of the housing.
 37. The methodaccording to claim 35, wherein the protective cover is formed by drawingaustenitic stainless steel.
 38. The method according to claim 35,wherein the hardness of the attachment portion is made lower byannealing.
 39. The method according to claim 38, wherein the annealingis performed by high-frequency induction heating.
 40. The methodaccording to claim 36, wherein the welding is laser welding.